It is known how to provide a load sensor based on a piezoresistive principle, using either strain gages attached to a stressed member, or integrated into a silicon chip. Such conventional sensors, however, are not only expensive, but also lack the robustness required in certain environments, such as in automotive and industrial applications.
It is also known to provide a sensor based on the Villari effect wherein a magnetostrictive material changes its magnetic permeability μ in response to variations in the applied stress (force).
In this regard, one type of configuration of such a strain sensor includes a conductive wire that is wrapped around a separate core member formed of magnetostrictive material. The strain sensor includes a ferromagnetic carrier that provides a return path for the magnetic flux outside of the wire coil. An air gap exists between the ferromagnetic carrier and the core member. An electrical current flowing through the wire coil generates a magnetic field that surrounds the wire and propagates partially within the core member. A strain applied to the core member changes the magnetic permeability therein. Inductance of the wire coil is a function of the permeability of the material through which the coils magnetic field flows. Thus, the strain applied to the core member changes the inductance of the wire coil. A drawback with such a strain sensor is that the air gap offers a permeability several orders of magnitude less than that of the core or the ferromagnetic carrier, so even a very small air gap significantly increases the magnetic flux reluctance. As a result, the sensitivity of the strain sensor is reduced. Further, manufacturing tolerances affect the size of the air gap during manufacture of the strain sensor, which results in inconsistent strain measurements by such sensors.
Accordingly, a practical design must comprise a complete magnetic circuit to obtain the required sensitivity and avoid the influence of external magnetic fields. As alluded to above, it is important to minimize air gaps in the path of the magnetic flux. Thus, two features of this type of force sensor that can make a practical implementation challenging are as follows: (1) both the load carrying and the load sensing functions are performed by the same part—the magnetostrictive shaft, which prevents independent optimization of each function; and (2) air gap minimization.
U.S. Patent Application Publication No. US 2004/0107777 A1 entitled “UNIVERSAL MAGNETOSTRICTIVE FORCE SENSOR” to Lequesne et al. discloses a magnetostrictive force sensor having a shell completely enclosing both the magnetostrictive shaft and the coil, which partially addresses the above challenge pertaining to air gap control. Even in view of this, however, challenges remain insofar as the load sensing function and load carrying function still coexist in the shaft (i.e., the load sensing component is in the load path, and thus must be big and durable enough to handle the load).
There is therefore a need for a strain sensor based upon the Villari effect that minimizes or eliminates one or more of the shortcomings set forth above.